Or is it? My first insight that maybe salt was not just salt came from a simple recipe. I was vexed because I couldn't get the amount of salt right. Rosemary red potatoes had a simple enough ingredient list (potatoes, butter, olive oil, rosemary and kosher salt), but I managed to screw it up half the time -- and it was always the amount of salt I would get wrong. My family would look like they just took a hit of rot gut whiskey. Trying to substitute table salt for kosher salt messed up the measurement (table salt has smaller crystals and is denser). Lesson: If the recipe calls for kosher salt and you don't have it, use less. The figure I've heard is approximately 25%, i.e. 1 T kosher = 2 1/4 t table salt.
My reference on issues like this is Cookwise by Shirley Corriher. She goes into more technical detail than I care to repeat here, but the lesson I take away from her is that all salts are not equal. Chemically, salt is salt, but how it is handled and refined leads to two important differences. First, the size and shape of the crystal will change how it adheres. The shape of the crystal may be different. It may just bounce. If it doesn't stick - your salt will be less salty because there is less of it.
The second difference is the refining. Unpleasant tasting trace minerals, particularly calcium and magnesium, are removed through successive stages of brining and evaporation. Counter intuitively, the lack of refinement has become a big part of marketing of special salts today - the presence of trace minerals are tauted for their health benefits. Those trace minerals will also lead to subtle differences in taste. If you want to capture the attention of a roomful of aspiring chefs, whisper "subtle differences." A not-so-new twist is to sell salt infused with seasonings and flavorings. Winning the prize for most ridiculous is Salt Works' Fusion "Lime Fresco Sea Salt, $11.99 for a 3.5 oz. jar of sea salt with lime. You better be using some nice tequila with those margaritas if your blowing 12 bucks on salt!
I finally broke down and bought a jar of sea salt with claims of better taste and beneficial trace minerals. I haven't found a use for it that made it stand out. I'll keep you posted. I am happy with the kosher salt I've been using for a few years. The consensus I've seen from a Cooks Illustrated survey, a Culinate article and others I've spoken with puts me in good company -- kosher is the best buy for the kitchen.
The hazard of writing about food is everyone has immediate food experience, and thousands are writing about it. About an hour after I started writing this piece, I saw an article in Culinate (linked above) by an engineer, attorney and I guess chef writing about salt. About the same time the New York Times covered children getting wigged out about "good" vs. "bad" food, featuring salt as a major culprit. I felt I got scooped.